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Swap it out for a ControlLogix PLC. OK, seriously, A cube root is the same thing X^(1/3). Unfortunately the 984LL exponent function EMTH(34) only support integer exponents. If you are computing the cube root of an integer and the integer has a limited range then use a table. Otherwise read the Wikipeida article on cube root - it has a few methods that you might be able to convert to 984LL, but its not going to be easy. Can you shove it off to your HMI, have the HMI computer X^.3333333 and then read it back? Good luck.

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this should be faster than Newton (2-3 iterations should be enough) b=a[ (a^3+R)+R] / [(a^3+R) + a^3] where b = current iteration result (root) a = previous result R= value we need to find cube root of for initial guess you can use a=R/2 or a=R/3